FISH GLUE (SIRIŠUM-E MĀHI)
Your statement is accurate. Fish glue, commonly known as isinglass, can indeed be extracted by heating the skin or bones of fish in water. The purest form of fish glue is derived from the membrane of the swim bladder, particularly from species like sturgeon. This method has been historically employed to obtain a high-quality adhesive.
Recipes:
Fish glue, known as sirišum-e māhi in Persian, is mentioned in several historical treatises, including works by Biruni (440 A.H./1048 A.D), Teflisi (6th A.H./12th century), and Majmu’at al-sanaye (1005 A.H. /1596 A.D) Jamali-ye Yazdi (M. 580 A.H./1184 A.D.), in Farrokhnameh, not only discusses the medical benefits of fish glue but also describes its use for writing on paper to achieve a golden appearance.
As a sizing material, fish glue is detailed in Resaleh-ye Joharrieh (837 AH/1433 A.D.), Resāle dar Bayān-e Kāḡaḏ Morakkab va Ḥall-e Alvān (Mid- 9th A.H./15th A.D), Golzār-e Ṣafā (950 A.H./1543 A.D.), Ḵaṭṭ va Morakkab (978 A.H./1571 A.D.), and Resāle dar Bayān-e Tariqe-ye Sāḵtan-e Morakkab va Kāḡaḏ-e Alvān (10th A.H., 16th A.D.), with almost identical descriptions. The first two sources outline the process as follows:
دیگر، قدری سریشم ماهی سفید را، سه شبانروز در آب پاک کند. وبعد از آن به اتش، نرم گرم کند: در حال شود بیالاید، وکاغذ بدان برآرد وبا احتیاط خشک کند ومهر زند وبنویسد.
Small amount of white- fish glue is soaked in clean water. Then it is heated on slow fire till it melts. Paper is sized with it and dried carefully followed by burnishing [it] to write.
Both sources, Resaleh-ye Joharrieh 837 AH/1433 A.D.) and Resāle dar Bayān-e Kāḡaḏ Morakkab va Ḥall-e Alvān (Mid- 9th A.H./15th A.D) specify that white fish should be used; this is found in the Caspian Sea and used to be very common in Iran.
The third and fourth sources, both Seyrafi (950 A.H./1543 A.D.) and Aqili Rostamdari (978 A.H./1571 A.D.) use the term mucilage (loʿāb) for gluey substance obtained from soaking fish glue. Ali Seyrafi describes the process as follows in his treatise Golzār-e Ṣafā :
صفت آهار سریشم ماهی
بعضی آهار بدین سان دادند فرقه ای رسم دگر بنهادند
زسریشم که بُوَد از ماهی داده آهار به خاطر خواهی
که سریشم سه شبانروز به آب بنهادند که تا گشت لعاب
نرم کردند به آتش در کار چونکه شد نرم نمودند آهار
Fish-glue as sizing
A kind of glue they gave in this fashion,
A sect devised a novel tradition.
From fish's isinglass, gleaming and grand,
They crafted glue for sizing, as planned.
Three nights it soaked in water's embrace,
To form a mucilage, with careful grace.
Softened by heat in a meticulous chore,
For sizing, the glue they did adore.
Hossein Aqili Rostamdari (978 A.H./1571 A.D.) describes the process in Ḵaṭṭ va Morakkab, as follows:
طریقی دیگر که از سریشم ماهی بود باید که سریشم را سه شبانه روز در آب نهد تا لعاب بدهد از آن او را در آتش نرم کند و کاغذ آهار دهد
The alternative method for sizing involves fish glue, Soaked for three days to extract mucilage, true. In the embrace of fire, it becomes soft and refined, To size the paper, a solution designed.
The fifth source, Resāle dar Bayān-e Tariqe-ye Sāḵtan-e Morakkab va Kāḡaḏ-e Alvān (10th A.H./16th A.D.), recommends the same process as mentioned above; however, it suggests filtering the fish glue after melting it on fire. Moreover, the recipe highlights that the same technique is applied for sizing with gum, most probably referring to gum arabic.
دیگر سریشم ماهی سه شبانه روز در آب کند بعد از آن به آتش گرم سازد تا در آب حل شود و به رگوی بیالاید، و کاغذ را بدان برآورد و در آفتاب خشک کند. بر وی خط نیکو آید. و آبِ صمغ نیز همین حکم را دارد
The other sizing involves fish glue, which is soaked in water for three days. Afterward, it is heated over a warm fire until it melts, then strained through muslin. The paper is sized with it and dried in the sun, resulting in a surface conducive to good writing. The same process applies to sizing with gum.
Experiment:
For sizing, isinglass proves to be a suitable option. The process begins by extracting air bladders from fish, ensuring thorough cleaning, and allowing them to air-dry. Subsequently, the dried bladder is carefully sliced into thin, translucent strips. These strips are then dissolved in hot water, followed by dilution and cooling to create flat disks.
Take the isinglass pieces and soak them overnight to soften and expand the dried material. The subsequent step involves slow cooking in a double boiler at 45 °C with continuous stirring. To enhance the solution, a small quantity of gum tragacanth, dissolved in water, can be added to serve as an emulsifier.
Once the solution has cooled, it is ready for sizing. Apply the glue smoothly on the paper using a brush. Alternatively, dip the paper into the solution, depending on its consistency. Allow it to dry and then burnish the paper for a smooth finish.